1. Field of the Invention
This invention is related to deformable optics for use with short wavelength radiation.
2. Background of the Invention
The use of mirrors or reflective optics for applications with short wavelength radiation (e.g., &lt;200 nm) is well-known. Reflective optics are required for short wavelength radiation because refractive optics absorb, but do not transmit at these wavelengths. Although reflective optics absorb some shortwave radiation, much of the incident radiation can be reflected with special reflective coatings.
One example of a commercial application for short wavelength radiation compatible mirrors is extreme ultraviolet (EUV) photolithography, which is used in the manufacture of semiconductor devices. The use of short wavelength or EUV radiation (&lt;20 nm) in semiconductor photolithography is being developed to reduce the resolution dimension, which is proportional to wavelength. A smaller resolution dimension allows more devices to be fabricated onto a given area of a wafer.
To accurately project an image onto a wafer through an optical system in photolithography, the image distortion must be very low. For commercial semiconductor production, the image distortion of an optical system should not be greater than 10% of the resolution dimension. Because the resolution dimension of EUV optical systems may be in the 50 to 100 nm range, the distortion must be proportionally small, approximately 5 to 10 nm. If the distortion exceeds 10% of the resolution dimension, excessive lithography errors can occur, resulting in defective integrated circuits.
In order to produce optical systems with less than 10 nm distortion, the mirrors used in EUV optical systems must have extremely high dimensional accuracy. Dimensionally correct mirrors have better imaging performance, higher EUV throughput, and longer lifetime between refurbishments. However, such low dimensional tolerances for optical systems make fabrication of acceptable mirrors, particularly aspheric mirrors, extremely difficult. Dimensional defects in mirror surfaces can result from inaccurate machining of the optics, and subsequent uneven deposition of reflective coatings, which are used to maximize the reflectivity of EUV optical elements. However, even with dimensionally perfect mirrors, dimensional defects can occur during short wavelength radiation exposure due to thermal loading, reflective coating stress, gravitational loading, and improper maintenance. Out of tolerance mirrors will produce unacceptably high image distortion at the wafer.
Deformable mirrors allow the reflective surface to be adjusted within the required dimensional tolerances during use without having to disassemble the optical system. U.S. Pat. No. 4,655,563 is an example of a deformable mirror which uses a number of actuators mounted on the back of the mirror to manipulate the mirror surface. Although the mirror of the '563 patent is deformable, it is not compatible with EUV radiation and therefore cannot deform the surface deflection within the nanometer tolerances required for short wavelength radiation photolithography optical systems.
Another deformable mirror is disclosed in U.S. Pat. No. 5,420,436. The '436 patent describes a deformable mirror that is compatible with EUV radiation and has a thin flexible reflective surface which is deformed by piezo actuators. Although the mirror described in the '436 patent is deformable, it also is not controllable within the tolerances required for EUV lithography, namely, nanometer to subnanometer accuracy. The actuators are connected between a stiff base and a flexible reflective surface, and any movement of the actuators directly deforms the reflective surface.
In view of the foregoing, there is a need for a deformable mirror which can precisely deform a mirror within the nanometer-subnanometer tolerances required for EUV lithography, and the reflective surface must be compatible with EUV radiation.